Vision impairments such as myopia (near-sightedness), hyperopia and astigmatism can be corrected using eyeglasses or contact lenses. Alternatively, the cornea of the eye can be reshaped surgically to provide the needed optical correction. Eye surgery has become commonplace with some patients pursuing it as an elective procedure to avoid using contact lenses or glasses to correct refractive problems, and others pursuing it to correct adverse conditions such as cataracts. And, with recent developments in laser technology, laser surgery is becoming the technique of choice for ophthalmic procedures. The reason eye surgeons prefer a surgical laser beam over manual tools like microkeratomes and forceps is that the laser beam can be focused precisely on extremely small amounts of ocular tissue, thereby enhancing accuracy and reliability of the procedure. These in turn enable better wound healing and recovery following surgery.
Hyperopia (far-sightedness) is a visual impairment where light entering the eye does not focus at the retina to produce a sharp image as desired, but rather focuses at a location behind the retina such that a patient sees a blurred disc. The basic principle to treating hyperopia is to add positive focusing power to the cornea. For instance, a hyperopic eye can be treated by placing a convex lens in front of the eye to add a positive focusing power to the eye. After correction, light passing through the convex lens and into the eye focuses at the retina to form a sharp image.
Different laser eye surgical systems use different types of laser beams for the various procedures and indications. These include, for instance, ultraviolet lasers, infrared lasers, and near-infrared, ultra-short pulsed lasers. Ultra-short pulsed lasers emit radiation with pulse durations as short as 10 femtoseconds and as long as 3 nanoseconds, and a wavelength between 300 nm and 3000 nm. Examples of laser systems that provide ultra-short pulsed laser beams include the Abbott Medical Optics iFS Advanced Femtosecond Laser, the IntraLase FS Laser, and OptiMedica's Catalys Precision Laser System.
Prior surgical approaches for reshaping the cornea include laser assisted in situ keratomileusis (hereinafter “LASIK”), photorefractive keratectomy (hereinafter “PRK”) and Small Incision Lens Extraction (hereinafter “SmILE”).
In the LASIK procedure, an ultra-short pulsed laser is used to cut a corneal flap to expose the corneal stroma for photoablation with ultraviolet beams from an excimer laser. Photoablation of the corneal stroma reshapes the cornea and corrects the refractive condition such as myopia, hyperopia, astigmatism, and the like.
It is known that if part of the cornea is removed, the pressure exerted on the cornea by the aqueous humor in the anterior chamber of the eye will act to close the void created in the cornea, resulting in a reshaped cornea. By properly selecting the size, shape and location of a corneal void, one can obtain the desired shape, and hence, the desired optical properties of the cornea.
In current laser surgery treatments that correct hyperopia using LASIK and PRK, positive focusing power is added to the cornea by steepening the curvature of the cornea, by for example, removing a ring-shaped stroma material from the cornea. In a LASIK procedure, a flap is first created, then lifted up for the ring-shaped stroma material to be removed or ablated away by an excimer laser. The center of the cornea is not removed while more outward portions of the cornea are removed. The flap is then put back into place. The cornea thus steepens due to the void created in the cornea. Common patterns that steepen the cornea include ring, tunnel and toric shapes. LASIK can typically correct hyperopia for up to 5D (diopter). In a PRK procedure where no flap is created, the epithelium layer is first removed, and the ring-shaped stroma material is then removed by an excimer laser. The epithelium layer will grow back within a few days after the procedure.
Recently, surgeons have started using another surgical technique other than LASIK and PRK for refractive correction. Instead of ablating corneal tissue with an excimer laser following the creation of a corneal flap, the newer SmILE technique involves tissue removal with two femtosecond laser incisions that intersect to create a lenticule for extraction. Lenticular extractions can be performed either with or without the creation of a corneal flap. With the flapless procedure, a refractive lenticule is created in the intact portion of the anterior cornea and removed through a small incision.
In the SmILE procedure illustrated in FIG. 10, a femtolaser 110 is used to make a side cut 120, an upper surface cut 130 and a lower surface cut 140 that forms a cut lens 150. A tweezer, for example, is then used to extract the cut lens beneath the anterior surface of the cornea 160 through the side cut 120. Recently, SmILE has been applied to treat myopia by cutting and extracting a convex lens-shaped stroma material with a femtosecond laser. However, SmILE techniques have not been applied in treating hyperopia.
Furthermore, as shown in FIG. 1, conventional femtosecond laser surgery systems generate a curved dissection surface to make a lenticular incision by scanning a laser focus on the intended dissection surface through a XY-scanning device and a Z-scanning device. This method does not use the more advantageous “fast-scan-slow-sweep” scanning scheme with femtosecond lasers having high repetition rate (“rep rate”), for e.g., in the MHz range. Using the “fast-scan-slow-sweep” scanning scheme for a lenticular incision, however, will generate vertical “steps” and will require many vertical side cuts, resulting in a lenticular dissection surface that is not smooth.
Therefore, there is a need for improved systems and methods to generate corneal lenticular incisions for high repetition rate femtosecond lasers to correct hyperopia.